Despite the high prevalence of biofilm-related oral diseases such as dental caries, there are no clinically effective therapies to disrupt virulent biofilms, resulting in >$40 billion expenditures annually in the US. Effective control of cariogenic biofilms is notoriously challenging because the bacteria are enmeshed in a protective extracellular matrix rich in exopolysaccharides (EPS). Furthermore, EPS-enmeshed bacteria create highly acidic microenvironments that promote acid-dissolution of tooth enamel, leading to the onset of dental caries. Current antimicrobial agents are incapable of disrupting the EPS matrix or affecting the physico- chemical aspects of caries and often fail to efficiently kill the microbes within biofilms, resulting in limited efficacy in vivo. To overcome these remarkable hurdles, we have developed an exciting therapeutic strategy using biocompatible iron oxide nanoparticles (IO-NP) with catalytic activity and pH-responsive properties that display both anti-biofilm and anti-caries actions. IO-NP exhibit peroxidase-like activity at acidic pH values that rapidly activates hydrogen peroxide (H2O2) in situ to simultaneously degrade the protective biofilm EPS-matrix and kill embedded bacteria with exceptional efficacy (>5-log reduction of cell viability) in 5 minutes. Moreover, IO-NP also reduce apatite demineralization in acidic conditions. We hypothesize that IO-NP synergizes with H2O2 to amplify anti-biofilm effects and prevent the onset of dental caries in vivo via nanocatalysis and enhanced in situ production of antibacterial, EPS-degrading and demineralization-blocking agents at acidic pH. The significance of this work is to develop a feasible and superior anti-biofilm and caries preventive approach compared to current chemical modalities. To test our hypothesis, we will optimize the efficacy of IO-NP/H2O2 to further improve anti-biofilm and demineralizing-blocking activities (Aim 1). We will enhance the catalytic activity of IO-NP by inclusion of specific metal salts into the nanoparticles, and explore the effects of various dextran- based coatings to increase IO-NP localization within biofilm structure. Furthermore, we will incorporate calcium-phosphate into IO-NP to enhance its effects on demineralization. Then, we will evaluate the efficacy of enhanced IO-NP/H2O2 for biofilm control in vitro using a mixed-species, cariogenic biofilm model (Aim 2). We will further elucidate the biological actions of IO-NP/H2O2 using time-lapsed confocal and biophysical methods to examine spatiotemporal degradation of EPS-matrix, bacterial killing and cohesiveness within intact biofilms. The effects on enamel demineralization will be assessed using micro-hardness and micro-CT. In Aim 3, we will evaluate the biocompatibility and efficacy of the developed IO-NP/H2O2 therapy in hindering cariogenic biofilms and the onset of carious lesions in vivo using a rodent caries model with a clinically-relevant topical treatment regimen. Successful completion of these aims will provide a framework for further formulation development and clinical efficacy studies. Importantly, IO-NP can be synthesized with low cost at large scale while H2O2 is readily available, which could lead to a feasible new anti-biofilm/anti-caries therapeutic platform for topical use.